The present invention relates to organometallic compounds and particularly to a method for making a complex of cerium (IV) with the .beta.-diketone dimethyl heptafluorooctanedione which exhibits unusually high volatility and stability.
Volatile metal complexes are of interest for a variety of applications, including use as fuel additives, metal vapor sources for vapor phase reactions, and gas transport reagents. A useful discussion of .beta.-diketonate complexes and their uses is provided by R. E. Sievers et al. in Science, 201 [4352], pages 217-223 (July 1978), wherein numerous references to the preparation and use of these complexes are cited.
.beta.-diketonates of the rare earth or lanthanide series of elements of the Periodic Table have the general formula Ln(AA').sub.3, wherein Ln is the metal element and AA' represents the diketonate ligand which forms the complex. Discussions of the synthesis and properties of the rare earth .beta.-diketonate complexes usually do not treat the cerium complexes. This is due to the complexity arising because cerium has two stable oxidation states.
The trivalent paramagnetic complexes of Ce.sup.III with .beta.-diketonates such as acetylacetone [Ce(acac).sub.3 ], trifluoroacetylacetone [Ce(tfa).sub.3 ], and hexafluoroacetylacetone [Ce(hfa).sub.3 ] are frequently mentioned but only a few tetravalent (Ce.sup.IV) compounds have been reported. These include Ce(acac).sub.4, Ce(tfa).sub.4 and complexes of cerium with 2,2,6,6-tetramethyl-3,5-heptanedione (thd) and the aromatic diketone (C.sub.6 H.sub.5 CO).sub.2 CH, the first two being made by the oxidative decomposition of trivalent complexes incorporating the same ligands in an inert solvent in flowing air or oxygen. In the case of Ce(tfa).sub.4, yields are poor even in the presence of excess quantities of the free .beta.-diketone (Hfta).
Cerium complexes have also been formed with some of the 10-carbon .beta.-diketones, including 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione [Ce(fod).sub.3 ] and 2,2,7-trimethyl-3,5-octanedione [Ce(tod).sub.4 ]. However, none of these cerium complexes have exhibited sufficient stability and volatility to be truly useful as a cerium metal vapor source for vapor phase reactions designed to produce cerium-containing products.
A new cerium complex exhibiting high volatility and good stability at volatilization temperatures is disclosed in my copending, commonly assigned patent application, Ser. No. 418,216 filed Sept. 15, 1982. This compound, referred to as Ce(fod).sub.4, is the complex of cerium (IV) with the fluorinated .beta.-diketone 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (abbreviated Hfod).
A process for making this diketonate complex, which is disclosed in my application, involves reacting the deprotonated diketone (fod.sup.-) with a cerium (III) compound such as Ce(NO.sub.3).sub.3.6H.sub.2 O to produce the octahedrally coordinated cerium (III) complex Ce(fod).sub.4.sup.-1, and then slowly oxidizing this complex by stirring under oxygen to produce the cerium (IV) complex Ce(fod).sub.4. A disadvantage of this process is that the oxidation step must be carried out slowly and completely in order to avoid residual cerium (III) complex in the product.